virgl: Clear the valid buffer range when possible
If we are discarding the whole resource in a synchronized way, the resource storage is now unused, either because we have created new resource storage, or because we have waited for the existing resource storage to become unused. We can thus mark the storage as uninitialized, but only if the resource is not host writable (in which case we can't clear the valid range, since that would result in missed readbacks in future transfers). Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>